Design and synthesis of highly twisted phenanthroimidazole substituted blue-emitting truxene based fluorescent chromophores

Article abstract of DOI:10.1039/c8nj04293d

The modified Debus-Radziszewski reaction was used to design and synthesize novel truxene derived blue-emitting materials bearing phenanthroimidazole substituents. The materials were prepared non-catalytically under ambient conditions using a triamino based derivative of truxene as a key precursor in good yields. The 5, 5′, 10, 10′, 15, and 15′ positions of the truxene were alkylated to provide solubility to the molecules, making the materials solution processable. The substituted phenanthroimidazole moieties were introduced at the 2, 7, 12 positions of truxene to synthesize the target molecules. Three new materials were prepared by varying the phenyl group attached at the C2 position of the phenanthroimidazole moiety, designated as PT₁ for the phenyl, PT₂ for the 4-methoxyphenyl, and PT₃ for the 4-cyanophenyl substituent. From the photophysical studies, PT₃ was found to have dominant charge transfer characteristics in its excited state. Also, PT₃ exhibited a photoluminescence quantum yield as high as 0.83. The optimized geometries from the computational studies revealed that the molecules are highly twisted, with dihedral angles of ～75°. The HOMO-LUMO distributions were found to be well separated showing the bipolar characteristic of the compounds. The band gap values from computational studies were found to be in agreement with experimental values. The emission spectra in the solid state gave maxima at 402 nm for PT₁, 412 nm for PT₂, and 450 nm for PT₃. The materials were thus found to be emitting in the blue region of the visible spectrum. The TCSPC and TRPL studies were carried out to explore further the excited states of the molecules, wherein radiative pathways were found to be dominant.

Full text of DOI:10.1039/c8nj04293d

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Design and synthesis of highly twisted
phenanthroimidazole substituted blue-emitting
Cite this:DOI: 10.1039/c8nj04293d
truxene based fluorescent chromophores†
b
a
Banpreet Kaur,a Dhanashree Moghe,b Dinesh Kabra
and Josemon Jacob
*
The modified Debus–Radziszewski reaction was used to design and synthesize novel truxene derived
blue-emitting materials bearing phenanthroimidazole substituents. The materials were prepared non-
catalytically under ambient conditions using a triamino based derivative of truxene as a key precursor in
good yields. The 5, 50, 10, 100, 15, and 150 positions of the truxene were alkylated to provide solubility to
the molecules, making the materials solution processable. The substituted phenanthroimidazole
moieties were introduced at the 2, 7, 12 positions of truxene to synthesize the target molecules. Three
new materials were prepared by varying the phenyl group attached at the C2 position of the
phenanthroimidazole moiety, designated as PT1 for the phenyl, PT2 for the 4-methoxyphenyl, and PT3
for the 4-cyanophenyl substituent. From the photophysical studies, PT3 was found to have dominant
charge transfer characteristics in its excited state. Also, PT3 exhibited a photoluminescence quantum
yield as high as 0.83. The optimized geometries from the computational studies revealed that the
molecules are highly twisted, with dihedral angles of B751. The HOMO–LUMO distributions were found
to be well separated showing the bipolar characteristic of the compounds. The band gap values from
computational studies were found to be in agreement with experimental values. The emission spectra in
the solid state gave maxima at 402 nm for PT1, 412 nm for PT2, and 450 nm for PT3. The materials were
thus found to be emitting in the blue region of the visible spectrum. The TCSPC and TRPL studies were
carried out to explore further the excited states of the molecules, wherein radiative pathways were
found to be dominant.
Received 22nd August 2018,
Accepted 20th December 2018
DOI: 10.1039/c8nj04293d
rsc.li/njc
of full-color displays and solid state lighting necessitates the
emission of all three primary colours: red, green and blue.
1. Introduction
In recent years, the design and development of p-conjugated However, in terms of stability, lifetime and colour purity, the
luminescent organic materials have gained considerable attention blue emitters are less preferred due to their intrinsic wide
from the scientific community due to their applications in organic band gap.3 Blue emitters have also been plagued by the issue
electronics including organic light emitting diodes (OLEDs), light of oxidative stability leading to keto defects in the fabricated
emitting electrochemical cells (LECs), piezoelectronics, etc. These devices.4,5 Several reports have been published on the development
materials have gained precedence over their inorganic counter- of fluorescent, phosphorescent6 and thermally activated delayed
parts, as their photonic and electronic properties can be easily fluorescence (TADF) blue emitter materials,6–8 with phosphorescent
tuned. The development of p-conjugated materials has been an and the TADF materials capable of realizing 100% internal quantum
active area of research since Tang demonstrated electro- efficiencies theoretically.9 However, both phosphorescent and TADF
luminescence in 1987,1 which was then extended to polymeric based materials have been found to have shorter lifetimes with large
materials by Friend and coworkers when poly( p-phenylene efficiency roll-offs at high brightness.10 Also, these materials need to
vinylene) was found to be electroluminescent.2 The attainment be dispersed into appropriate host materials to avoid emission
quenching, thus creating the need for the development of suitable
host materials, or else unfavourable host–guest systems might lead
a Department of Materials Science and Engineering, Indian Institute of Technology
to phase separation diminishing the device performance.11 This
Delhi, Hauz Khas, New Delhi 110016, India. E-mail: jacob@polymers.iitd.ac.in;
in turn leads to complex device architecture, bringing up the
overall cost of the device. Apart from that, the phosphorescent
materials are based on noble and rare earth elements (e.g.
Fax: +91 11 2659 1421; Tel: +91 99 1106 1890
b Department of Physics, Indian Institute of Technology Bombay, Powai,
Maharashtra 400076, India
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c8nj04293d iridium, ruthenium, etc.), making their commercial production
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very expensive. Considering the drawbacks of the phosphorescent
and TADF materials, the traditional fluorescent materials seem
like a viable and attractive option, leading to architecturally
simpler devices.12 Since the blue emitting materials lie near the
high energy end of the visible spectrum, one of the coherent
approaches has been to incorporate high energy bulky moieties.
Tuning the band gap and maintaining the twisted geometries of
bulky moieties to limit molecular packing has been found to
improve EQEs.10,12–14 The twisted geometry reduces the conjuga-
tion, keeping the systems near the higher energy end, and thus
assists in achieving deep blue to blue emissions. Also, the twisted
geometries prevent intermolecular interactions and aggregation
in solid state which usually leads to either colour impurities or,
worse, quenching of emission in the solid state. The resulting
non-coplanar structures could subdue the crystallization in
solid state.14
According to this strategy we have designed and synthesized
three new molecules based on truxene–phenanthroimidazole
conjugates for blue emission. Truxene based materials have
been reported as blue emitting in nature.15–17 The availability of
facile positions for substitution provides ease of chemical
tailoring. Positions 5, 50, 10, 100, 15, 150 can be easily substituted
by alkyl chains, imparting solubility in common organic solvents
and therefore making the materials solution processable.16
Positions 2, 7, 12 have been utilized for tuning of the band gap
corresponding to blue emission.18 However, truxene is planar in
nature and bulky groups are required since aggregation in the
solid state might lead to colour impurities.17 This C3 symmetric
molecule due to its easy chemical tailoring, thermal stability and
solution processability is a highly sought after core to design
p-conjugated materials.19 There are several reports where truxene
based materials have been used in OLEDs, in OPVs and as
fluorometric sensors.15,20,21 The side moiety of the reported
conjugates, phenanthroimidazole (PIM) is a nitrogen-containing,
imidazole heterocycle based group with a rigid planar structure.
PIM has been used as the side moiety for the design of novel
materials as it has been reported as an ideal building block for
deep blue/violet emissions, due to its excellent charge transport
properties, thermal stability, and high photoluminescence
Fig. 1 Structures of the phenanthroimidazole substituted truxene conjugates,
PTx.
Also, varying the aryl groups attached at the C2 position of PIM
could be used to tune the electronic properties judiciously, provide
effective steric hindrance and even improve PLQY.22,26,27 The effect
of different aryl substituents at the C2 position of the PIM moiety
has been investigated by incorporating electron withdrawing and
electron donating groups (CN, OMe) and further evaluation of the
photophysical, electrochemical and computational characterization
to gain insights into the structure–property relationship.
2. Experimental details
2.1. Materials
quantum yield (PLQY).22 PIM based molecules, by virtue of The precursors and chemicals used for the synthesis were
the nitrogen atoms in the imidazole ring, possess an amphoteric procured from Merck, Sigma Aldrich, CDH, and spectrochem
characteristic, with one nitrogen being electron deficient and the and were used without further purification. The dry solvents
other one being electron rich. It shows donor properties when an used for the synthesis and photophysical studies were procured
electron withdrawing group is attached to the imidazole ring and from Merck and Sigma Aldrich and were kept in the glove box. The
acceptor properties in the presence of an electron donating starting material, truxene (1), was synthesized from 1-indanone
group.10,23,24 A large number of PIM based small molecules have following the procedure mentioned in the literature.28 Molecules
been reported,25–27 but there are no reports on star-shaped 5,5,10,10,15,15-hexahexyl-10,15-dihydro-5H-diindeno[1,2-a:10,20-c]-
truxene–PIM conjugates.
fluorene (2), 5,5,10,10,15,15-hexahexyl-2,7,12-trinitro-10,15-di-
In this report truxene–PIM conjugates were prepared by a hydro-5H-diindeno[1,2-a:10,20-c]fluorene (3) and 5,5,10,10,15,15-
modified Debus–Radziszewski27 reaction, utilizing commercially hexahexyl-10,15-dihydro-5H-diindeno[1,2-a:10,20-c]fluorene-2,7,12-
available aldehydes in the presence of acetic acid and ammonium triamine (4) were synthesized according to the literature with
acetate (Fig. 1). The materials were synthesized non-catalytically slight modifications.18,29,30 Silica gel of mesh size 60–120 mm
and in good yields using a simple methodology convenient for procured from Merck was used for column chromatography.
scale-up. The side moiety of PIM was attached to truxene core from The reactions were monitored by thin layer chromatography
the N1 position of the imidazole ring. It has been reported that (TLC), which was performed on silica gel pre-coated sheets
the N1 position of PIM can be used to extend the conjugation. (Merck, 60G, F254).
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2.2. Synthetic procedure
PT1. The compound was purified by column chromatography
using hexane/dichloromethane (3 : 7) as eluent (0.35 g, 84.9%).
M.P. = 232 1C. 1H NMR (400 MHz, CDCl3): d 8.91 (3H, d,
J = 7.6 Hz, PIM-H), 8.81 (3H, d, J = 8.5 Hz, PIM-H), 8.74 (3H,
d, J = 8.4 Hz, PIM-H), 8.53 (3H, d, J = 8.8 Hz, PIM-H), 7.77 (3H, m,
truxene-H), 7.70–7.66 (12H, m, PIM-H), 7.54–7.49 (6H, m,
PIM-H), 7.46–7.41 (3H, m, truxene-H), 7.32–7.28 (6H, m, PIM-H),
7.25–7.21 (3H, m, PIM-H), 7.19–7.15 (3H, m, truxene-H), 3.04–2.98
(6H, m, CH2), 2.14–2.04 (6H, m, CH2), 1.16–0.98 (36H, m, CH2),
0.89–0.71 (30H, m, hexyl chain-CH2CH3). 13C NMR (100 MHz,
CDCl3): d 156.01, 151.29, 146.96, 141.29, 138.18, 137.89, 137.77,
130.86, 129.80, 129.72, 129.17, 128.58, 128.49, 128.45, 128.35,
127.73, 127.61, 127.49, 126.20, 126.13, 126.04, 125.25, 124.60,
123.44, 123.12, 121.18, 56.59, 37.11, 32.08, 30.02, 29.77, 24.72,
24.49, 22.70, 14.31. HRMS (ESI) m/z calculated for C126H127N9
(M + H)+ 1724.0122, found 1724.0256.
2.2.1. 5,5,10,10,15,15-Hexahexyl-2,7,12-trinitro-10,15-dihydro-
5H-diindeno[1,2-a:10,20-c]fluorene 3. To a solution of 5,5,10,10,
15,15-hexahexyl-10,15-dihydro-5H-diindeno[1,2-a:10,20-c]fluorene
(1.50 g, 1.77 mmol) in dichloroethane (200 mL) was added
fuming nitric acid (1.47 mL), and the mixture was stirred at 0 1C
for 3 hours. The reaction mixture was then poured onto ice, and
the pH was neutralized by adding sodium hydroxide solution. The
solution was then washed several times with ethyl acetate to extract
the compound; the organic part was then collected and dried over
sodium sulphate. The solvent was evaporated to obtain the crude
product. The product was recrystallized with ethyl acetate to afford
yellow coloured solid (1.51 g, 95.8%). 1H NMR (400 MHz, CDCl3): d
8.54 (3H, d, J = 8.6 Hz, truxene-H), 8.38–8.35 (6H, m, truxene-H),
2.96–2.88 (6H, m, CH2), 2.27–2.19 (6H, m, CH2), 0.97–0.80 (36H, m,
CH2), 0.62–0.45 (30H, m, hexyl chain-CH2CH3). 13C NMR
(100 MHz, CDCl3): d 155.20, 149.87, 147.43, 145.54, 137.77,
125.17, 122.95, 117.87, 56.98, 37.12, 31.68, 29.53, 24.35, 22.49,
14.09. Anal. calcd for C63H87N3O6: C, 77.02; H, 8.93; N, 4.28.
Found: C, 77.32; H, 9.23; N, 4.31.
2.2.2. 5,5,10,10,15,15-Hexahexyl-10,15-dihydro-5H-diindeno-
[1,2-a:10,20-c]fluorene-2,7,12-triamine 4. To a solution of 3 (1.50 g,
1.53 mmol) in ethyl acetate (76.5 mL), tin chloride dihydrate
(10.35 g, 45.90 mmol) was added and the resulting mixture was
refluxed overnight in the presence of a nitrogen atmosphere.
Upon completion of the reaction, it was cooled to room temperature.
The reaction was quenched with a saturated solution of sodium
bicarbonate. The solution was filtered, and the filtrate was
extracted with ethyl acetate. The organic part was collected
and dried over sodium sulphate, and the solvent was removed.
The crude product was obtained as a brown coloured solid. The
crude product was purified by column chromatography using
hexane/ethyl acetate (9 : 1 v/v) as an eluent. Upon purification, the
product was obtained as a pale yellow solid (1.31 g, 96.4%). 1H NMR
(400 MHz, CDCl3): d 8.06 (3H, d, J = 8.4 Hz, truxene-H), 6.75 (3H,
d, J = 1.7 Hz, truxene-H), 6.69 (3H, dd, Ja = 8.3 Hz, Jb = 2.2 Hz,
truxene-H), 3.76 (6H, br, NH2), 2.87–2.80 (6H, m, CH2), 2.04–
1.85 (6H, m, CH2), 0.96–0.81 (36H, m, CH2), 0.63–0.45 (30H, m,
hexyl chain-CH2CH3). 13C NMR (100 MHz, CDCl3): d 155.84,
144.78, 141.21, 138.14, 132.48, 125.47, 113.38, 109.13,
55.24, 37.36, 31.74, 29.77, 24.05, 22.49, 14.06. Anal. calcd for
C63H93N3: C, 84.79; H, 10.50; N, 4.71. Found: C, 83.95; H, 10.68;
N, 4.38.
PT2. The compound was purified on silica gel by column
chromatography using hexane/ethyl acetate (5 : 1) as eluent
1
(0.37 g, 87.1%). M.P. = 242 1C. H NMR (400 MHz, CDCl3): d
8.90 (3H, d, J = 7.2 Hz, PIM-H), 8.80 (3H, d, J = 8.4 Hz, PIM-H)
8.73 (3H, d, J = 8.4 Hz, PIM-H), 8.56 (3H, d, J = 8.4 Hz, PIM-H),
7.76 (3H, m, PIM-H), 7.72 (3H, m, truxene-H), 7.69–7.63 (9H, m,
PIM-H), 7.55–7.48 (6H, m, PIM-H), 7.39 (3H, d, J = 8.6 Hz,
truxene-H), 7.15 (3H, m, truxene-H), 6.78 (6H, d, J = 8.8 Hz, PIM-
H), 3.75 (9H, s, O–CH3), 3.02–2.95 (6H, m, CH2), 2.18–2.10 (6H,
m, CH2), 1.09–0.99 (36H, m, CH2), 0.78–0.61 (30H, m, hexyl
chain-CH2CH3). 13C NMR (100 MHz, CDCl3): d 160.45, 156.05,
151.36, 146.99, 141.31, 138.24, 137.93, 137.72, 131.13, 129.59,
128.57, 128.35, 128.10, 127.71, 127.62, 127.18, 126.20, 125.95,
125.34, 125.08, 124.54, 123.88, 123.49, 123.17, 123.10, 122.18,
121.10, 114.49, 113.82, 56.63, 55.54, 37.22, 32.10, 30.04, 29.82,
24.72, 24.49, 23.03, 14.31. HRMS (ESI) m/z calculated for
C
129H133N6O3 (M + H)+ 1814.0439, found 1814.0738.
PT3. The compound was purified over silica gel by column
chromatography using hexane/ethyl acetate (5 : 1) as eluent
1
(0.35 g, 82.1%). M.P. = 278 1C. H NMR (400 MHz, CDCl3): d
8.87 (3H, d, J = 8.4 Hz, PIM-H), 8.82 (3H, d, J = 8.8 Hz, PIM-H),
8.74 (3H, d, J = 8.5 Hz, PIM-H), 8.62 (3H, d, J = 8.6 Hz, PIM-H),
7.85 (6H, dd, Ja = 8.2 Hz, Jb = 5.1 Hz, PIM-H), 7.79 (3H, m,
truxene-H), 7.76–7.71 (6H, m, PIM-H), 7.65–7.58 (3H, m,
truxene-H), 7.55–7.51 (9H, m, PIM-H), 7.36 (3H, t, J = 8.7 Hz,
PIM-H), 7.19–7.13 (3H, m, truxene-H), 3.09–3.03 (6H, m, CH2),
2.21–2.11 (6H, m, CH2), 1.16–0.91 (36H, m, CH2), 0.82–0.71
(30H, m, hexyl chain-CH2CH3). 13C NMR (100 MHz, CDCl3): d
156.50, 148.57, 147.39, 141.75, 138.30, 138.22, 137.62, 135.17,
132.09, 130.22, 129.87, 129.25, 128.84, 127.99, 127.58, 127.37,
126.54, 125.89, 124.80, 123.60, 123.31, 123.09, 121.21, 118.44,
112.68, 56.86, 37.29, 32.07, 29.85, 24.77, 22.74, 14.31. HRMS
(ESI) m/z calculated for C129H124N9 (M + H)+ 1798.9979, found
1798.9860.
2.2.3. General synthetic procedure for PTx. 9,10-Phenan-
threnequinone (0.212 g, 1.02 mmol) was added to a solution of 4
(0.300 g, 0.34 mmol) in acetic acid (1.35 mL). Then, ammonium
acetate (0.285 g, 3.69 mmol) and aldehyde (1 equivalent,
1.02 mmol) were added to the solution. The reaction mixture
was refluxed and the progress of the reaction was monitored by
TLC. After the completion of the reaction, it was quenched with
water. The solution was then filtered and washed with methanol
several times. The precipitate obtained was then dissolved in
2.4. Instrumentation
chloroform and washed with water several times to remove 1H NMR and 13C NMR spectra were obtained at 400 MHz and
traces of impurities. The organic part was collected and dried 100 MHz, respectively, on a Bruker NMR spectrometer. The
over sodium sulphate, and the solvent was removed to obtain high-resolution electrospray ionization (ESI) mass spectrometric
the crude compound.
measurements were taken on a Waters SYNAPT G2 spectrometer.
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The spectra were recorded in deuterated chloroform. UV-vis spectra the materials were obtained from a Biologic-SP 150 electro-
were obtained using a PG Instruments’ T90+ UV-visible spectro- chemical quartz crystal microbalance at a constant scan rate
photometer. The steady-state photoluminescence spectra were of 100 mV sꢀ1 using the supporting electrolyte of 0.1 M tetra-
recorded on an Agilent Cary Eclipse fluorescence spectrophoto- butylammonium perchlorate in dichloromethane. These experi-
meter. The excitation and the emission slits were 5/5 nm for the ments were performed at room temperature using a three
excitation measurements. The spectra in solution were recorded electrode cell, with glassy carbon as the working electrode,
using THF keeping the concentration of the solution at 10ꢀ5 M. calomel electrode in saturated KCl solution as the reference
The solid state spectra of the molecules were recorded in the electrode and platinum wire as the counter electrode.
thin film form by spin-coating the films on a quartz substrate,
2.5. Theoretical studies
using a solution of the compounds dissolved in chloroform
at 2500 rpm. The films were annealed and the absorption
and emission spectra were recorded. The photoluminescence
quantum yield of the materials was recorded by a relative
method in THF using anthracene as the standard (FF = 0.27 in
ethanol).31 The time-correlated single photon counting (TCSPC)
was recorded on a spectrometer (Horiba Jobin Yvon IBH, UK) in
THF solution with the excitation at 295 nm, the time resolution
of 7 ps, and the instrument response function (IRF) of 324 ps.
The fluorescence measurement was carried out under deaerated
conditions. The solutions were prepared in anhydrous solvents
in a nitrogen atmosphere in a glove box and sealed in a cuvette.
The decay curves were fitted utilizing the monoexponential
decay equation on IBH DAS v6.2 software. For time-resolved
photoluminance (TRPL), the 355 nm laser line of Picolo AOT
InnoLas pulsed laser (1 kHz) with a pulse width of 840 ps
was used as the excitation source and the delayed emission
was collected with an Andor-gated iCCD with a minimum gate
The electronic properties of the synthesized materials were
further investigated by computational methods, for which the
frontier molecular orbital analysis was carried out utilizing the
B3LYP method and 6-31G(d,p) level of density functional theory
(DFT) by Gaussian 09.32 The structures were optimized, and
frequency calculation of the optimized structures was carried
out. The fully optimized structures were found to have no
imaginary frequencies. The time-dependent DFT (TDDFT) cal-
culations were carried out with CAM-B3LYP/6-31G(d,p) in THF
solvent using the IEFPCM model for solvent effects. For the
simplification of the calculation, the hexyl groups in the molecules
were replaced by methyl groups.
3. Results and discussion
3.1. Synthesis
width of B700 ps and variable gate delay, to capture intensity The synthetic pathway followed to obtain the target molecules is
variation of B9 orders of magnitude. Cyclic voltammograms of shown in Scheme 1. The core moiety, truxene 1, was synthesized
Scheme 1 Reaction scheme for the synthesis of the target molecules.
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from 1-indanone in the presence of hydrochloric acid and acetic The TDDFT studies revealed the first transitions taking place in
acid. The truxene was then rendered soluble in common organic the molecules, where in the case of PT3, MO patterns also confirm
solvents by alkylation with n-hexyl bromide, using n-BuLi as a the presence of CT characteristics (Fig. S4, ESI†). The DFT studies
base and THF solvent, at its 5, 50, 10, 100, 15, 150 positions to (vide infra) revealed that the optimized structures of the molecules
give hexa-alkylated truxene derivative 2. The nitration of 2 and possessed highly twisted geometries. The unsubstituted target
the subsequent reduction of the tri-nitro derivative 3 led to the key molecule PT1 was the most blue-shifted amongst all. The absorption
precursor 4. Subsequently, this precursor 4 was used to synthesize spectra of the compounds were also recorded in solvents of varying
the target molecules by the modified Debus–Radziszewski reaction polarities. The recorded spectra and the maxima values are shown
in good yields (475%). The 9,10-phenanthrenequinone, on reaction in Fig. S1 and Table S1 (ESI†), respectively. There was no significant
with 4 in the presence of the respective aldehydes (benzaldehyde for shift recorded in the absorption maxima values of the compounds,
PT1, 4-methoxybenzaldehyde for PT2, and 4-cyanobenzaldehyde for indicating that there is no significant interaction of the solvents
PT3) in acetic acid, resulted in three new target molecules. The with the ground state of the compounds. The absorption spectra of
compounds were found to be soluble in common organic the thin films were found to be similar to the solution spectra apart
solvents such as chloroform, dichloromethane, tetrahydrofuran, from the expected broadening of the peaks. The shoulder peak at
chlorobenzene, etc. All the compounds were purified by column 365–370 nm still had maximum intensity in the case of PT3. Also,
chromatography. The purified compounds were characterized the absorption edge of PT3 was red-shifted as compared to PT1 and
by NMR (1H, 13C) spectroscopy and mass spectrometry.
PT2, indicating that PT3 has the smallest optical band gap amongst
the three molecules. This could be ascribed to the presence of the
electron withdrawing cyano group in the phenyl ring at the C2
3.2. Photophysical properties
The absorbance and the steady-state photoluminescence spectra position of the PIM moiety. The optical band gaps of PT1, PT2, and
of the synthesized molecules were obtained in THF solution PT3 were determined from the onset of the absorption to be 3.27 eV,
(10ꢀ5 M) and in thin films spin-coated on a quartz substrate. 3.24 eV, and 3.03 eV, respectively.
The spectra in solution and thin film are shown in Fig. 2 and 3,
In solution, the steady-state photoluminescence of the synthe-
respectively, and the data are summarized in Table 1. An intense sized materials gave an emission maximum in the deep blue/violet
peak at around 250–260 nm was observed in the case of all the region for PT1, whereas PT2 and PT3 emitted in the blue region of
three molecules, which could be attributed to the p–p* transitions the visible spectrum. The molecules PT1 and PT2 show vibronic
in PIM units and benzene ring.33 The same has also been coupling, whereas PT3 shows a broad and featureless spectrum. A
confirmed by the TDDFT studies carried out (vide infra) where similar vibronic feature was obtained in the solid-state emission
the electronic transitions show high energy maxima at 248 nm for spectra of PT1 and PT2. The emission spectra in the film form were
PT1 and PT2 and at 253 for PT3 (Table 2) with the oscillator found to be red-shifted, even with the presence of long hexyl
strength ( f ) values corresponding to 1.7487 (PT1), 1.4798 (PT2), chains and bulky groups like PIM as the side moieties in the target
and 0.8910 (PT3). Another peak at around 315 nm was sharp in the molecules.17,36 The spectrum of PT2 was found to be most red-
case of PT1 and PT2 and was weaker and broadened in the case of shifted in the solid state. This might be due to the presence of
PT3. This peak could be ascribed to the p–p transition of the PIM strong intermolecular forces amongst the methoxy groups of the
fragment.34,35 A smaller shoulder peak was observed at around methoxy-substituted phenyl ring attached to the C2 carbon of PIM
365–370 nm for all three target molecules. The intensity of the in the solid state. The introduction of the cyano group in PT3
peak was much higher in the case of PT3, which could be ascribed resulted in the most red-shifted absorption and emission spectra
to the charge transfer (CT) state. However, in the case of PT1 and overall, with an emission maximum of PT3 at 450 nm.37 The
PT2, the shoulder peak was found to be of weaker intensity due to solvation effect on the emission spectra of the three molecules was
the absence of and weaker CT states, respectively.27 The CT state also recorded in solvents of different polarities. The solvents used
peak was also broadened and showed vibronic structure for PT3. were toluene, chloroform, THF and N,N-dimethylformamide (DMF),
Fig. 2 Absorbance spectra of the target molecules (a) in THF solution and (b) in thin film.
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Fig. 3 Steady-state emission spectra of the target molecules (a) in THF solution and (b) in thin film (lex = 350 nm).
Table 1 The optical properties of the synthesized PIM–truxene based materials
Solutiona
Thin film
abs
max
emis
max
abs
max
emis
a
b
d
Compound
l
(nm)
l
(nm)
l
(nm)
l (nm) ledge (nm) tF (ns) FF
max
krc (ꢂ108 sꢀ1
)
knr (ꢂ108 sꢀ1
)
PT1
PT2
PT3
261, 316, 359 386
259, 315, 364 396
254, 312, 365 421
256, 315, 364 412
258, 315, 368 438
243, 316, 371 450
379
382
409
6.28
5.34
2.48
0.50 ꢁ 0.02 0.79
0.57 ꢁ 0.02 1.07
0.83 ꢁ 0.04 3.35
0.79
0.80
0.68
a
b
c
d
Measured in THF solution. Measured in solution using anthracene as a standard (FF = 0.27, in ethanol). kr = FF/t. FF = kr/(kr + knr).
in the order of their increasing polarities. The molecules PT1 processes in the deactivation of the excited state can be evaluated
and PT2 showed vibronic structures in their emission in low by calculating the radiative (tr) and non-radiative (tnr) decay time
polarity as well as high polarity solvents. However, PT3 showed constants, tr = (tF/FF) and tnr = tF/(1 ꢀ FF).25 The presence of
broad featureless emission spectra even in solvents with low different substituents at the phenyl ring attached to the C2
polarity showing a positive solvatochromism with the increase position in the PIM moiety did not affect the value of tnr much
in solvent polarity, indicating a stabilized excited state in polar with values ranging from 12.4 to 14.58 ns. However, the values of
solvents (as shown in Fig. 4). This also confirms that the charge tr were affected significantly. PT1 gave a value of 12.6 ns, whereas
transfer (CT) characteristic dominates in the excited state of when the methoxy group was introduced, the value decreased
PT3.27,38,39 The phenomenon is explained more by the DFT to 9.36 ns. With cyano group substitution, there was an
computations as well, in the next section. The PLQY of the approximately fourfold decrease in the value of radiative decay
compounds was recorded by using anthracene as a standard, time constant, with a value of tr equaling 2.98 ns.
and the FF values were found to be 0.50 ꢁ 0.02, 0.57 ꢁ 0.02, and
The TRPL spectra of the materials were recorded in THF
0.83 ꢁ 0.04 for PT1, PT2, and PT3, respectively. The introduction solution and are shown in Fig. 6. For PT1 and PT2, there was an
of electron donating groups usually leads to an increased PLQY, order drop in intensity within 10 ns with the prompt lifetime of
as observed in the case of PT2.13 However, the PLQY has rarely B7.59 ꢁ 0.05 ns and 4.8 ꢁ 0.04 ns, respectively. In the case of
been found to increase with the incorporation of electron with- PT3, a two order drop in the intensity was observed within 10 ns
drawing groups like –CN.26 It has been explained by the model with the prompt fluorescence lifetime of B2.09 ꢁ 0.04 ns. The
based on supermolecular interactions among –CN groups and sudden jump in the intensity of PT3 in 10–20 ns range is due to
the phenanthrene plane. The increase in the PLQY of PT3 could the artefact of the laser line. The values of the prompt lifetime
be attributed to this rare phenomenon. TCSPC was performed of the excited state of the materials are comparable with those
to study the lifetime of the excited states of the molecules. The obtained from TCSPC. In the region well away from the prompt,
results are shown in Fig. 5, and the excited state lifetimes are the PT1 delayed emission (measured at 407 nm) can be fitted
listed in Table 1. The emission was measured at 360 nm and using a biexponential fit with a lifetime of B110 ns (15%) and
380 nm for PT1, at 380 nm and 400 nm for PT2 and at 420 nm for 12.6 ns (84%) indicative of emission from two different excitonic
PT3. A mono-exponential decay was observed in all the three species. In the case of PT2, beyond 100 ns, the data can be fitted
target molecules. The lifetime of PT1, PT2, and PT3 was found to using the power law where the intensity is Btꢀn with n being
be 6.38 ns, 5.09 ns, and 2.48 ns, respectively. The mono- B2.8. In the case of dilute solutions, the power law could
exponential decay with lifetime values ranging from 2 to 6 ns originate because of a solute–solvent interaction that leads to
reflects the rigid nature of the compounds.13 The radiative broadening of the density of states (DOS).40 Further, the power
pathways were found to be more dominant in the case of PT2 law can also originate because of the contribution from triplets or
and PT3, as evident from the calculated values of kr and knr polaron pairs.41 For the polaron pair, the power factor (n) is B1
(Table 1). The contribution from the radiative and non-radiative and for triplets n is B2. Since the power term in our case is 42,
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Table 2 Computed vertical transitions, oscillator strengths and contributions
in the compound obtained by TDDFT (CAM-B3LYP/6-31G) in THF solvent
Wavelength
Compound (nm)
Oscillator
strength ( f )
Major contribution (%)
PT1
PT2
PT3
305
277
248
HOMO - LUMO+2 (50.4%)
HOMOꢀ1 - LUMO+1 (24.7%)
HOMOꢀ1 - LUMO+3 (3.5%)
HOMOꢀ2 - LUMO (46.1%)
HOMOꢀ4 - LUMO (17.6%)
HOMOꢀ2 - LUMO+5 (2.8%)
1.6874
1.5201
HOMOꢀ3 - LUMO+2 (21.4%) 1.7487
HOMOꢀ4 - LUMO+1 (4.4%)
HOMOꢀ2 - LUMO+1 (4.0%)
307
278
248
HOMO - LUMO+2 (52.8%)
HOMOꢀ1 - LUMO+1 (25.2%)
HOMOꢀ1 - LUMO+3 (2.7%)
HOMOꢀ2 - LUMO (38.1%)
HOMOꢀ4 - LUMO (14.1%)
HOMOꢀ1 - LUMO (4.4%)
1.9515
1.2168
Fig. 4 Emission spectra of PT3 in solvents with different polarities.
HOMOꢀ3 - LUMO+2 (13.8%) 1.4798
HOMOꢀ4 - LUMO+2 (7.7%)
HOMOꢀ2 - LUMO+2 (5.9%)
highly twisted geometries. The twisted geometries of the molecules
prevent it close packing and are likely to suppress the quenching
of the fluorescence in the solid state.25 The calculated highest
occupied molecular orbital (HOMO) shows electron density
concentrated at the side moieties of the phenanthroimidazole
groups, and the lowest unoccupied molecular orbital (LUMO)
shows electron density predominantly at the truxene core of the
molecule, as can be seen in Fig. 10. The HOMO and the LUMO
are well separated in the case of all the three target molecules.
This shows the bipolar nature of the compounds and provides a
balanced charge transfer for the emitter in devices.10,43,44
This also confirms the CT characteristic of the excited state of
PT3 theoretically. The band gaps in the gaseous phase were
calculated from the HOMO and the LUMO levels. The values
were found to be 3.52 eV, 3.43 eV and 3.54 eV for PT1, PT2, and
PT3, respectively. The value of the band gap of PT3 was found to
be highest in the case of the theoretical band gap, whereas PT3
gave the lowest value of optical band gap amongst all three
materials. This could be attributed to the CT state of PT3, as the
CT interactions decrease the band gap value.10 The discrepancy
between the measured and estimated value of band gap could
be because of the functional used. Since CT excitations often
require the inclusion of exchange energies, range-separated
hybrid functional may be required for the correct prediction
of the ground state electronic properties.45 The band gap values
of PT1 and PT2 were found showing a trend similar to that
obtained experimentally. However, the TDDFT calculations of
the molecules were carried out at the CAM-B3LYP/6-31G level
which is a hybrid exchange–correlation functional. The HOMO–
LUMO band gap values (Table 3) were obtained from the energy
328
277
HOMOꢀ1 - LUMO+1 (28.3%) 3.1062
HOMO - LUMO (53.6%)
HOMOꢀ4 - LUMO+2 (23.0%) 1.7601
HOMOꢀ2 - LUMO+2 (36.4%)
HOMOꢀ2 - LUMO (7.3%)
253
HOMOꢀ4 - LUMO (24.8%)
HOMOꢀ2 - LUMO+1 (10.0%)
HOMOꢀ2 - LUMO (5.1%)
0.8910
we would expect the broadening of DOS as a major contribution
causing non-exponential dependence. The delayed fluorescence
for PT3 can be fitted with triexponential (three excitonic species)
fit with a lifetime (amplitude) of 28.1 ns (6.6%), 4.8 ns (92.7%),
108.2 ns (0.7%). The emission spectra as a function of delay show
no spectral shift (Fig. 8), implying the absence of migration to any
low-energy tail states in the PT3 chromophore. Both the prompt
and delayed emissions show a linear dependence on the laser
fluence along with a linear dependency of delayed emission on
prompt emission (Fig. 7). The absence of spectral shift and linear
dependency of emission could indicate thermal activation of
excitons to emissive levels. However, the fast (B100s of nano-
second) decay of delay emission in solution suggests otherwise.
Decay lifetime of a CT emission is expected to be longer com-
pared to that of non-CT emission.42 The CT characteristics of PT3
due to electron withdrawing group (CN) may be responsible for
the stronger charge separation, thereby lowering the CT states
such that prompt or delayed emission takes place from this
lowest CT states and not shallow trap states.
3.3. Computational studies
The geometry optimization of the synthesized compounds was of the lowest singlet excited state transition from the TDDFT
performed using the Gaussian 09 software performing density calculation and were found to have a much better correlation
functional calculations at the B3LYP level using 6-31G(d,p) as with the experimental values, following the trend observed
the basis set. The calculations were carried out in the gaseous experimentally.46
phase and the optimized geometries are shown in Fig. 9. The
The TDDFT computations were performed to obtain the vertical
compounds were found to have large dihedral angles, 74.61, transition energies of the possible excitations, their respective
74.51, and 75.21 for PT1, PT2, and PT3, respectively, showing contributions and the oscillator strength. The values obtained
that the compounds are non-planar in nature and possess from these computations are given in Table 2. In the case of PT3,
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Fig. 5 Time-correlated single photon counting for the synthesized molecules in THF solution for the excited state lifetimes.
Fig. 6 Time-resolved photoluminescence spectra of the compounds (a) PT1, (b) PT2, and (c) PT3 in THF (10ꢀ5 M) solution.
the lowest energy transition was obtained at 328 nm corres- values corresponding to these transitions were observed at
ponding to a HOMO - LUMO transition, which corresponds to 316 nm and 315 nm for PT1 and PT2, respectively.
the experimental value of 365 nm. In PT1 and PT2, the lowest
3.4. Electrochemical properties
energy transition was observed at 305–307 nm due to the PIM
fragment (Fig. S4, ESI†), with significant contribution from the The electrochemical properties of the target molecules were
HOMO - LUMO+2 transition, corresponding to the oscillator studied by cyclic voltammetry (CV). The cyclic voltammograms
strength of 1.6874 and 1.9515, respectively. The experimental obtained are as shown in Fig. S3 (ESI†). The curves obtained
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Fig. 7 (a) Fluence dependence of prompt fluorescence in PT3. (b) Fluence dependence of delayed fluorescence in PT3. (c) Delayed fluorescence vs.
prompt fluorescence; PF was measured at the delay of 0 ns, and DF was measured at delay of 200 ns.
Fig. 8 Emission spectra of PT3 as a function of delay show no shift.
were found to be irreversible in nature, with two subsequent
oxidation processes. The value of the HOMO was determined
from the ionization potential of the materials using eqn (1). The
ionization potential was in turn obtained from the onset of first
oxidation in the CV curve. The onset of oxidation for the
Fig. 10 HOMO LUMO distributions for the synthesized molecules.
materials was found to be in the range of 0.65–0.75 V. The
LUMO values were calculated from eqn (2) utilizing the values
of the optical band gap and HOMO obtained from CV. The
HOMO and LUMO values of the compounds are listed in
Table 3.
EHOMO = ꢀ(Eox + 4.4) eV
ELUMO = EHOMO + Egopt
(1)
(2)
Fig. 9 Optimized geometries of the synthesized molecules, obtained from DFT calculations at B3LYP/6-31G(d,p).
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Table 3 Electrochemical properties of the target molecules
Eogpt (eV)
EHOMO (eV)
ELUMO (eV)
Eg (eV)
Eg (eV)
a
a
b
b
b
c
Compound
EHOMO (eV)
ELUMO (eV)
PT1
PT2
PT3
ꢀ5.14
ꢀ5.11
ꢀ5.06
ꢀ1.84
ꢀ1.87
ꢀ2.02
3.27
3.24
3.03
ꢀ5.29
ꢀ5.11
ꢀ5.63
ꢀ1.78
ꢀ1.68
ꢀ2.09
3.52
3.43
3.54
4.06 (3.29d)
4.03 (3.10d)
3.77 (2.96d)
a
b
c
Obtained experimentally from cyclic voltammetry. Obtained from DFT calculations using B3LYP/6-31G(d,p). Obtained from TDDFT calcula-
tions using CAM-B3LYP/6-31G(d,p). Vertical optical gap from TDDFT.49
d
Fertilizers (grant no. 15012/1/2012-PC-1), Department of Science
and Technology (grant no. TMD/CERI/BEE/2016/035/G), as well
as the UGC, India (fellowship to BK), for the financial assistance.
We would also like to acknowledge Prof B. Jayaram (Coordinator,
The lowering of LUMO was observed in the case of PT3, due
to the electron withdrawing cyano group present in the mole-
cule. Also in PT3, both HOMO and LUMO levels were found
lowered when compared with the theoretical HOMO–LUMO
SCF-Bio and Professor, Department of Chemistry, IITD, India) for
levels resulting in a lesser value of the band gap; this might
providing the super-computational facility to carry out com-
indicate the role of solvent interaction with the molecule PT3, as
putational studies. The authors are thankful to DST-AISRF and
Prof. Anindya Dutta, Department of Chemistry, IITB, for TCSPC
characterization. DM would like to acknowledge IIT Bombay,
India, for fellowship.
the DFT computational studies were carried out in the gaseous
phase.47 In the case of PT2, the HOMO level was observed to
have an increased energy value when compared to PT1, due to
the presence of electron donating methoxy group in the aryl ring
at the C2 position of PIM. Also, lowering of the LUMO level of
PT2 should lower the band gap value as well, which was
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